The dehydrogenation of hydrocarbons is an important commercial process. This is because of the great demand for dehydrogenated hydrocarbons as feedstocks for industrial processes. For example, dehydrogenated hydrocarbons are utilized in the manufacture of detergents, high octane gasolines, pharmaceutical products, plastics and synthetic rubbers, and various other products. One example of a specific dehydrogenation process is dehydrogenating isobutane to produce isobutylene, which may then be polymerized to provide tackifying agents for adhesives, viscosity-index additives for motor oils and impact-resistant and anti-oxidant additives for plastics. Isobutylene may also used in the manufacture of Methyl Tertiary Butyl Ether (MTBE) or may be dimerized to isooctane. MTBE and isooctane are used as anti-knocking agents in gasoline fuels.
The major traditional sources of alkenes have been steam cracking, and fluid catalytic cracking. Both processes simultaneously provide a wide range of other products with limited flexibility. However, as the demand grows for specific alkenes, such as isobutylene or propylene, it is not cost effective to satisfy that demand utilizing expensive cracking units. The best technical choice for providing alkenes has been the normal dehydrogenation of alkanes.
Major factors affecting the technical implementation of alkane dehydrogenation are (1) the thermodynamic equilibrium limiting single pass conversion as well as (2) the endothermic character of the dehydrogenation reaction. Additionally, the temperature required to carry out conventional dehydrogenation causes thermal cracking which lowers alkene selectivity, especially in the case of propane dehydrogenation.
Two types of catalysts based on chromium oxides for the dehydrogenation of lower alkanes have been described in the scientific and patent literature: chromium oxides supported on γ,δ/,θ-alumina doped with alkali metal, and chromium oxides supported on ZrO2. Chromium oxides supported on γ,δ/,θ-alumina doped with alkali metal are employed in the Lumnus Catofin and Snamprogetti-Yarsintez fluidized bed dehydrogenation processes. As well, chromium oxides supported on ZrO2 have been investigated for their higher thermal stability relative to alumina.
The precise nature of the active sites in chromium oxide-supported catalysts has been the subject of scientific debate for many years. As is known, Cr2O3 is the most stable form of all of the possible chromium oxides.
Two types of Cr6+ species have been detected in chromium oxide supported on alumina, after the calcination treatment and before reaction. It is generally believed that after reduction with hydrogen, all Cr6+ is reduced to Cr3+. However, according to Grunert et al. (J. Catal. 110 (1986), 138), the reduction occurs in two steps: an initial very rapid step from Cr6+ to Cr3+, followed by a slower step from Cr3+ to lower oxidation states (Cr2+). The active sites of the dehydrogenation reactions have been assumed to be Cr3+ by Grunert W. et al. (J. Catal. 99 (1986), 149; Delmon B. et al., J. Catal. 24 (1972), 336) Konig P. et al. (Acta Chim. Acad. Sci. Hung. 76 (1976), 123), both Cr2+ and Cr3+ by Ashmawy F. M. (J. Chem. Soc., Faraday Trans. 76 (1980), 2096), or coordinatively unsaturated Cr2+ by Lunsford H. et al. (J. Catal. 91 (1985), 155).
The catalytic activity in the dehydrogenation of isobutane as well as of ethane has been found to be proportional to the chromium content, whichever the species present. Also, in the case of propane dehydrogenation over ZrO2 supported chromium oxide (Indovina et al., Appl. Catal. 81 (1992), 113), the activity per atom of chromium was found to be the same for all chromium loadings. Such activity was attributed to the presence of mononuclear Cr3+ species.
However, since hydrogen is present as part of the dehydrogenation reaction product, further reduction of Cr3+ will continue. It is shown in this invention that the lower oxidation state chromium species are responsible for undesired cracking reactions taking place during dehydrogenation and thus to be responsible for coke formation. To minimize the formation of cracking products and coke, the oxidation state of chromium catalytic sites must be controlled by means of a functional redox system during the reaction cycle. Such a redox system according to the present invention is believed to be Cr2+/Cr3+/CO2, as is demonstrated hereinbelow by the addition of carbon dioxide to the hydrocarbon in the feedstock.
It is known from the literature that oxygen can shift the thermodynamic equilibrium of dehydrogenation reactions towards increased olefin production by reacting with the hydrogen product. Oxygen is also believed to combust coke and thereby keep the catalyst surface clean of coke deposits. Regeneration of the catalyst is thereby avoided, since coke does not build up and consequently the catalyst is not deactivated. Such reactions are called oxidative dehydrogenation reactions. Up to date there is no commercial process available for producing light alkanes, especially C3 and C4-olefins, by oxidative dehydrogenation reactions due to the absence of a proper catalyst. Known oxidative dehydrogenation reaction processes are described in several published patents.
For example, U.S. Pat. No. 4,996,387 disclosed a dehydrogenation process with a continuous regeneration of dehydrogenation catalyst achieved by cyclically contacting a portion of the catalyst with an admixture of oxygen containing regeneration gas and diluent, while contacting the remaining portion of the catalyst with an admixture of hydrocarbon feed material and diluent. In this disclosure oxygen is added for the purpose of catalyst regeneration in a separate regime from the dehydrogenation medium.
Canadian Patent 912,051 describes a vapor phase process for dehydrogenation of paraffin and olefins with oxygen and halogen in the presence of a solid catalyst containing an alkali metal or an alkaline earth metal compound and a promoter. U.S. Pat. No. 3,697,614 is directed to olefin production by oxidative dehydrogenation using a molten alkali metal hydroxide containing alumina with an in solution transition metal oxygenation, preferably consisting of dichromate molybdate, tungstate, manganate, permanganate, ferrate and metavanadate.
U.S. Pat. No. 4,046,833 disclosed a vapor phase process for dehydrogenation of paraffinic hydrocarbon containing 3 to 6 carbon atoms to the corresponding monoolefin, wherein the process is carried out in the presence of oxygen and in the presence of an oxidative dehydrogenation catalyst containing vanadium and aluminum. The effective paraffin to oxygen ratio is claimed to be in the range of 1:0.04 to 1:10. Examples given in the disclosure demonstrate only the effect of several catalyst compositions tested under different conditions with different oxygen concentrations.
U.S. Pat. No. 4,788,371 discloses another oxidative dehydrogenation process using a dehydrogenation catalyst comprising at least one noble metal component. According to the disclosure, an alkane feed including an oxygen containing gas is introduced into a dehydrogenation reactor containing the noble metal catalyst. The oxygen added to the reactor feed is in a molar ratio to alkane which is very similar to that of U.S. Pat. No. 4,046,833. The effect of oxygen on the described system is to combust hydrogen while minimizing the combustion of valuable hydrocarbons.
Numerous processes have also been disclosed which involve the removal of hydrogen from a mixture of hydrogen and one or more organic compound. For example, U.S. Pat. No. 4,788,371, and other patents referenced therein, disclose a process for the dehydrogenation of hydrocarbons in which hydrogen obtained by the dehydrogenation of the hydrocarbons is catalytically reacted with oxygen. A disadvantage of each of these processes is that some of the oxygen gas reacts chemically with the organic compounds instead of the hydrogen, thus converting them into undesired products.
European Patents A1-0219271 and A1-0219272 also disclose processes for the dehydrogenation of hydrocarbons in which hydrogen obtained from such dehydrogenation is removed. In these processes, the dehydrogenation takes place in the presence of a zeolite catalyst, and the hydrogen is removed by chemical reaction with an acidic oxide such as sulfur dioxide or nitrous oxide. The processes do not share the disadvantage of processes which utilize oxygen gas because sulfur dioxide and nitrous oxide are not as reactive as is oxygen towards the organic compounds. They also appear to be less effective in the removal of hydrogen.
The oxidative dehydrogenation reaction processes described above have certain drawbacks. They generally suffer low olefin selectivity and currently are not utilized in commercial scale due to the lack of a suitably active and selective catalyst. A further disadvantage of all of these processes is the fact that using oxygen as a promoter of dehydrogenation reactions requires special handling. Oxygen can form explosive mixtures with hydrocarbons, and thus expensive explosion protection apparatus is generally required to safely utilize it on a commercial scale.
In the production of olefins by the catalytic dehydrogenation of paraffins, it is of course desirable to obtain as a high yield of olefin as possible in a single conversion pass. To minimize hot spot effects, and consequently increase the life of the catalyst, it is also desirable to conduct the reaction under such conditions wherein a minimum amount of coke is formed on the catalyst. It is also desired to increase the dehydrogenation reaction period by decreasing coke formation. Finally, it is desired to accomplish these goals using a “friendly” and safe nonexplosive reaction promoter, whose use does not require any investment in special safety equipment.